The fetal cardiovascular responses to acute and chronic stress will be studied in chronically catheterized fetal monkeys. Changes in heart rate and blood pressure will be recorded directly. Biochemical changes will be measured on micro-blood samples. Radionuclide-labelled microspheres will be used to study changes in blood flow distribution. The effect of CNS active drugs on fetal breathing movements will be studied by means of fetal tracheal pressure recordings. The fetal uptake of glucose and oxygen, and rate of urea production will also be investigated.